The invention relates to an integrated GPS (Global Positioning System) navigational system for space applications and specifically for space vehicles and satellites. The invention also relates to associated navigational processes.
For on-board independent determination of navigation data of satellites, especially for determination of position, velocity and GPS time of the satellite, GPS navigation systems or GPS receivers are used. Depending on the position at which the satellite equipped with the GPS receivers is located relative to the GPS satellite constellation (orbiting altitude about 20,000 km), it is necessary to take into account whether the satellite orbit is below the GPS satellite constellation (low earth orbit, LEO), or whether the satellite orbit is above the GPS satellite constellation and normally in geostationary earth orbits (GEO, about 30,000 km).
In the case of LEO satellites, navigation is possible using the standard process for determination of position by GPS. For this purpose, at least four signals from four different GPS satellites to the GPS receiver are measured and processed in parallel channels and then the position (three unknowns) and the clock bias, i.e. the difference relative to a time reference value (one unknown) are calculated. Because of the geometric conditions at low orbits, four or more GPS satellites are normally always visible. The accuracy of determination of the position depends on the geometric distribution of the visible GPS satellites. A measure of this accuracy is a xe2x80x9cdilution of precisionxe2x80x9d value (DOP), which is defined only for four or more satellites. This determination is usually also used for navigation planning, i.e. for defining the GPS satellites that will be used for determination of the satellite position, and the position error due to amplification of raw-data measurement errors.
Alternatively, sequential estimation methods (filter methods such as with Kalman filters) are also used with LEO-satellites for position determination. Thereby chronological measurements are combined with each other by means of dynamic models of the orbital movement and the on-board clock, so that, in the cases of LEO-satellites, these estimation methods lead to clearly improved estimates. For LEO-applications with sequential estimation methods, GPS-receivers are also usually used with four or more parallel channels, so that with this estimation method the DOP-measure can be used for also evaluating the navigational solutions, i.e. in particular for monitoring geometric independence of the measurement data.
The use of GPS in geostationary orbit with GEO-satellites has not heretofore been achieved. With GEO-satellites the use of sequential estimation methods is necessary, because in this case the GPS data cannot normally be received from four GPS-satellites simultaneously, and chronological measurements are necessary. Nevertheless, the poor geometric distribution of the GPS-satellites for the GEO-orbit, causes the estimates to greatly depend on the modeling uncertainty of the on-board clock. In the publication by S. Averin, V. Vinogradov, N. Ivanov, V. Salischev xe2x80x9cOn Combined Application of GLONASS and GPS Systems in Conditions of Limitedxe2x80x9d (in xe2x80x9cObservability of Navigation Satellitesxe2x80x9d, ION GPS 96, page 287 ff.) a possibility of how to avoid these difficulties is described. Therein, instead of using running time measurements, differences of running time measurements are used as a measured variable. Thereby the clock-bias, or, time-difference value of the on-board clock is limited and the dynamic model for the estimation filter now consists of only an orbit dynamics characteristic, with relatively high accuracy, and it no longer contains the relatively uncertain clock model.
The construction of a conventional GPS receiver usually comprises four components: an antenna, an HF (high frequency) front-end system (pre-amplifier, down converter, A/D converter), a digital signal processor, which, in particular, performs a correlation for determining carrier phases and code phases and comprises special digital electronic modules (ASICs), and a navigational processor (frequency lock means, phase lock means, delay means, means for decoding the navigational data, navigational planning means, and position determination means).
GPS navigational devices are currently available in the market. These devices have a disadvantage in space application in that they cannot be shared with other software programs of an on-board computer of the satellite. Consequently, LEO-satellites must be provided with an individual computer (CPU), a program and data memories, an on-board clock, a power supply as well as software modules, particularly for error monitoring and correction in addition to the other systems normally provided in LEO satellites. Consequently, the weight, performance, complexity and cost of the complete system are increased. Additionally, when estimating the satellite position by means of sequential data processing (filtering), there is limited or no access to information in the on-board computer of the satellite and which could be used to improve the estimated position. This information relates, for example, to accurate models of perturbing forces acting on the satellite, caused by thrust of the satellite drive means, or satellite-specific data corresponding to solar radiation pressure. The access to this data is particularly important if it is available during satellite operation, (for example, satellite mass).
A further disadvantage of the known devices is that a considerable part of the signal processing (correlation) of a GPS receiver is realized by means of electronic hardware. Radiation resistant electronic components, as used in space application, however, are very expensive and raise the cost of a GPS receiver, or are not even available or have to be specially produced.
An object of the invention is to provide a GPS navigational system on board a LEO or GEO satellite, which shares as many functions and resources with the devices already on board the satellite, and satisfies the usual requirements with respect to radiation resistance. The navigational system should also comprise means for determining the GPS system time.
An advantage provided by the invention is that a large part of the signal processing means, in particular the correlation of the GPS signal and the carrier signal, are realized by means of software. Consequently, the use of expensive, special radiation resistant electronic components (ASICs or Specific Integrated Circuits) is avoided.
In order to minimize computer requirements, according to the invention, the number of parallel processing channels is kept as small as possible, and less than four channels is preferable. This means, however, that a reduced number of GPS satellites need be visible, even for orbits below the altitude of the GPS orbit (approximately 20,000 km). Hence, measurement data from a restricted number of GPS satellites are used. Accordingly, position determination of the satellite is not directly possible but only by means of sequential estimation methods (filter process). However, this is not detrimental since the filter processes mainly rely on orbit models and disturbing forces, for example, thrust forces which are very well known and are mainly provided as software programs in the on-board computer of an orbit and attitude control system. Additionally, according to the invention, for measuring the transit time of a GPS (pseudorange) signal, it is regularly switched between all or most of the visible GPS satellites, so that geometric independent measurement data is obtained.
A further feature according to the invention, for improving the position estimates is that information updated in the on-board computer can be accessed, for example, external thrust forces generated by the satellite drive units. This possibility is provided, according to the invention, by the integration of the GPS receiver into the computer on-board the satellite. Accordingly, relevant information such as time of thrust activation, thrust direction and calibration parameters of the drive means normally supplied to the system are fed to the on-board computer.
For determining and monitoring the quality of the measurement geometry, i.e. the distribution of the GPS satellites, which are used for position determination, the conventional DOP-factor can no longer be used, since it is only defined for at least four GPS receivers which are simultaneously visible. However, according to the invention, the DOP-measurement can be replaced by a modified DOP-measurement, namely by taking into account not only the geometric distribution of the GPS satellites, but also their distribution in time and nominal movement of the receiver.
The invention provides a GPS navigational system for a satellite in space which comprises a front end section having an input for receiving GPS signals from a plurality of GPS satellites, a digital preprocessor device connected to the front end section for digitally reprocessing the GPS signals received from the front end section, and a signal processor connected to the digital preprocessor for decoding the GPS signals to determine the position of the satellite. An on-board computer is provided and a first data bus line connects the data preprocessor and the signal processor for bi-directional data exchange therebetween and a second data bus line connects the signal processor and the on-board computer for bi-directional data exchange therebetween.